The present invention relates to the use of a polymeric release composition for molds and formers used in the production of natural and synthetic rubber articles. In particular the polymeric release composition is useful for the formation of latex gloves, and also for preventing blocking on the outer surface of such gloves. The release composition of the present invention has the advantage of being easily cleaned from molds and formers, and is compatible with coagulant currently used in many manufacturing processes, eliminating the need for an extra step to coat the mold or former with a release coating.
As used herein, the terms latex glove or latex article refer to a glove or article made of natural or synthetic rubber. Articles made from natural or synthetic rubber are elastic materials having low glass transition temperatures. The surfaces of these materials are tacky and tend to adhere to each other. For example, latex gloves are difficult to strip from glove formers at the end of the manufacturing process, and they tend to stick together, or block, when packaged for distribution and sale.
There are currently several approaches to solving the problem. One method involves the use of a particulate or powder material, such as starch, talc, or calcium carbonate. The particulate can be blended into a coagulant solution to form a barrier between the latex rubber and the surface of the former. The powder facilitates the stripping of gloves from the formers, and also prevents blocking. Unfortunately, the powder coating is a known nuisance, as loose powder can become airborne. Starch powder tends to adsorb proteins found in natural rubber latex and the powder is easily dislodged during use, contaminating the surrounding environment and causing allergies and other negative effects. Further, the protein/powder complex serves as a food source for bacteria, allowing them to proliferate. Recently, there has been a growing demand for powder-free natural and synthetic rubber gloves, which do not use loose powder.
Another approach is the use of a chlorination process to provide the necessary anti-blocking properties, as described in U.S. Pat. No. 4,851,266. In this case, calcium carbonate is used as a mold release agent and washed away prior to chlorination. Although this reduces the tack and friction of the rubber, this process makes the rubber less pliant and reduces the shelf life of the rubber article. Also, chlorination leaves the article quite slippery, which makes it difficult to handle objects with chlorinated gloves. To address this problem, chlorinated gloves are often textured.
Yet another approach is the use of silicone materials. These materials facilitate removal of gloves from the formers, and also reduce blocking. Unfortunately, the residue of this material is rather hard to clean from the former in preparation for the next dipping cycle.
Talc-free mold release agents using surfactants are described in U.S. Pat. No. 4,310,928.
Polymeric mold release agents have also been disclosed in the art. Formulated polychloroprenes are described in EP 0 640623, Urethane dispersions in U.S. Pat. No. 5,534,350, and Styrene/acrylates containing silicone in U.S. Pat. Nos. 5,993,923; 5,691,069; 5,700,585; and 5,712,346. Copending U.S. patent application Ser. No. 09/400,488, and copending U.S. Patent Application, submitted Sep. 15, 2000 describe the use of star polymers as inner coatings for latex gloves.
Surprisingly it has been found that a release composition containing a water-borne high Tg polymer formed from at least one hydrophobic monomer and at least one hydrophilic monomer provides a powder-free mold release agent, provides anti-blocking in the finished article, and also aids in providing a smooth latex deposition on the formers. Additionally, the release composition is dispersible in high electrolyte coagulant, making it compatible with current manufacturing processes.
The present invention is directed to a mold or former for natural or synthetic rubber articles having on its surface a release composition comprising a water-borne polymer formed from at least one hydrophobic monomer and at least one hydrophilic monomer, where the polymer has a Tg of at least xe2x88x9210xc2x0 C.
Other embodiments of the invention are methods of making a latex glove in which a release composition, containing a water-borne polymer, having a Tg of at least xe2x88x9210xc2x0 C., and formed from at least one hydrophobic monomer and at least one hydrophilic monomer, is applied to a former as a release coating.
Still another embodiment of the invention is a natural or synthetic rubber article having on its surface a coating comprising a polymer having a Tg of at least xe2x88x9210xc2x0 C., and formed from a hydrophobic monomer and a hydrophilic monomer.
Still another embodiment of the invention is a latex coagulant, comprising a release composition, comprising a polymer having a Tg of at least xe2x88x9210xc2x0 C., and formed from a hydrophobic monomer and a hydrophilic monomer, which is used in manufacturing powder-free rubber articles.
While not wishing to be bound to any particular theory, it is believed that the key performance characteristics desirable for a release coating are: a high Tg polymer, which provides anti-blocking; ease of cleaning from a former; wettability of the film, for smooth latex deposition; an affinity to the latex surface; the ability to impart an anti-blocking character to the article formed from a mold or former.
The present invention is directed to a mold or former for the production of natural and synthetic rubber articles, having on its surface a release composition comprising a water-borne high Tg polymer formed from at least one hydrophobic monomer and at least one hydrophilic monomer.
Natural and synthetic rubber articles, as used herein, refer to articles made from low-Tg, tacky polymeric materials. Examples of such materials include, but are not limited to, butyl rubber, natural latex rubber, polyvinyl chloride, neoprene, nitrile, viton, styrene butadiene copolymers, polyurethanes, or interpenetrating polymer network emulsion polymers, or combinations of these.
The water-borne polymer of the present invention is one which is water-borne, and formed by means known in the art, such as emulsion polymerization and suspension polymerization.
By high Tg polymer is meant a polymer having a Tg of at least xe2x88x9210xc2x0 C., preferably from 25 to 200xc2x0 C., and most preferably from 40 to 150xc2x0 C. Monomers useful in forming the polymer of the present invention are ethylenically unsaturated monomers or mixtures thereof. Particularly useful hydrophobic monomers include (meth)acrylates, vinyl acetate, ethylene, and styrene. A preferred hydrophobic monomer is styrene.
The polymer of the present invention is also formed from a hydrophilic monomer. The hydrophilic monomer is present in the polymer at from 10-90 percent, based on the weight of the polymer. Suitable hydrophilic monomers include those monomers that are ionic, e.g. anionic, cationic, or zwitterionic, or have sufficient nonionic polar functionality, e.g. hydroxyl or amido groups to render them hydrophilic. Examples of such monomer include, but are not limited to hydroxyethyl acrylate, acrylonitrile, 2-(dimethylamino)ethyl (meth)acrylate, [3-(methacryloylamino)propyl]trimethylammonium chloride, 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt (a.k.a. AMPS), [2-(acryloyloxy)ethyl]trimethylammonium chloride. Preferred hydrophilic monomers are acrylic acid and methacrylic acid.
The polymer may optionally contain a cross-linker. The cross-linker is present at from 0 to 10 percent, and preferably from 0.5 to 5 percent by weight, based on the weight of the polymer. The cross-linker is preferably a di-functional cross-linker, such as divinylbenzene, diallyl maleate, ethylene glycol dimethacrylate, vinyl crotonate and diallyl phthalate. Multi-functional cross-linkers, such as allyl and vinyl sucrose ethers, pentaerythritol tetra(meth)acrylate, trimethylolpropane tri(meth)acrylate, and the like, can also be used.
The emulsion polymer of the present invention has an average particle size as measured by photon correlation spectroscopy in the range of from 0.05 to 1.5 microns, preferably from 0.15 to 0.9 microns, and most preferably from 0.2 to 0.6 microns. The particle size may be adjusted by means known in the art such as polymer seeding, initiator concentration, control of the salinity, water-hardness, surfactant concentration, type of surfactant, and by temperature effects on the surfactant solubility and initiator decomposition rate. It has been found that the particle size of the emulsion polymer is critical for proper anti-blocking properties. The particle size is also important to coagulant resistance. Particles that are too small have a poor resistance to a coagulant, while particles that are too large can settle in a dilute coagulant solution.
The release composition may optionally contain other ingredients, such as dispersants, surfactants, microspheres, and rheology modifiers.
Surfactants useful in the present invention include, but are not limited to: anionic, cationic, nonionic, and amphoteric surfactants; and polymeric surfactants including but not limited to linear and star copolymers. The surfactant improves the uniformity of the latex deposition, improves the ease of stripping and improves the wettability of the coated former by the latex.
Preferably the release coating contains no silicones, however the addition of a silicone to the composition can also be advantageous. A problem with having silicon compounds in the release composition is that they can remain on a mold or former, making the mold or former more difficult to clean.
The release composition may also contain microspheres. Microspheres are useful in reducing the surface contact area, and thus the adhesion between the coated rubber article and the mold, former, or other object. This improves both the release and anti-blocking characteristics. The microspheres have diameters below 60 microns, preferably from 5 to 40 microns, and most preferably from 10 to 30 microns. The microsphere may be made of any material that is harder than the article being coated. Examples of microspheres useful in the present invention are those made of polyamides such as nylons, polymethylmethacrylate, polystyrene, polyethylene, polypropylene, polytetrafluoroethylene, polyesters, polyethers, polysulfones, polycarbonates, polyether ether ketones, and other polymers and copolymers, silica, and microcrystalline cellulose. Preferably the microspheres are present in the release composition at from 0.05 to 5 percent by weight, and most preferably at from 0.1 to 1 percent by weight.
A dispersant may optionally be added to aid in dispersion of the microspheres into the aqueous release composition. Dispersants useful in the present invention include, but are not limited to, surfactants and polymeric dispersants including amphiphilic linear and star copolymers.
A rheology modifier is optionally present in the release composition. The rheology modifier is used to control the viscosity of the composition for ease of use in different manufacturing processes and equipment, and to control the uniformity and thickness of the coating. Rheology modifiers useful in the present invention include, but are not limited to cellulosics such as hydroxyethylcellulose, cationic hydroxyethylcellulose, such as polyquaternium-4 and polyquaternium-10, hydrophobically modified hydroxyethylcellulose, carboxymethylcellulose, methylcellulose, and hydroxypropylcellulose; dispersed or soluble starches or modified starches; and polysaccharide gums such as xanthan gum, guar gum, cationic guar gum such as guar hydroxypropyltrimonium chloride, and locust bean gum. Other suitable rheology modifiers include but are not limited to acid soluble copolymers, surfactants, etc. The rheology modifier is typically added at from 0.01 to 10 percent by weight, and preferably from 0.02 to 3 percent by weight, based on the release composition.
The release composition may also contain other additives known in the art, such as adhesion promoters, crosslinking agents, biocides, low surface energy compounds, fillers, and anti-foaming agents.
The release composition of the present invention is made by combining each of the ingredients to form an aqueous dispersion, by means known in the art.
The polymeric release coating may be used to coat a variety of materials used as molds and formers of natural and synthetic rubber articles. The molds and formers may be of any material known in the art, including but not limited to ceramics, glass, and stainless steel. Items produced in the molds or formers include gloves, prophylactics, catheters, tires, swimming caps, balloons, tubing, and sheeting. A particularly suitable end use application is as a release composition in the production of latex gloves, including surgeons"" gloves, physicians"" examining gloves, and workers"" gloves, more particularly powder-free latex gloves.
When used to coat the molds or formers in the preparation of gloves, the polymeric coating composition may be applied using several different methods. For example, in one method for coating the glove mold or former, a former or mold in the shape of a hand is dipped into a solution or dispersion containing the release composition, then dipped into a coagulant mixture containing a latex coagulant. Useful coagulants include, but are not limited to, calcium nitrate and calcium chloride. In another method, a clean former or mold in the shape of a hand is dipped into a coagulant containing a release composition. After drying, the mold coated with the release coating and coagulant can be used to prepare a glove by methods known in the art. For example, the coated mold is immersed in a natural or synthetic rubber latex for a time sufficient for the rubber to coagulate and form a rubber coating of the desired thickness. Optionally, the glove then may be water leached to remove impurities from the rubber. The formed glove is then oven cured, cooled, and dipped into a coating composition, which will become the inner glove coating. Alternatively, the formed glove can be dipped into a coating composition prior to the oven cure and cooling. The inner surface treatments include, but are not limited to those known in the art, including polymers such as that described in U.S. patent application Ser. No. 09/663,468 incorporated herein by reference, other polymer coatings, chlorination, and starch or clay powders. After cooling, the glove is simultaneously stripped from the mold and inverted. Some, or all of the release coating on the glove mold or former is transferred to the what has become the outer surface of the glove.
The release composition and the coagulant can be combined into the same solution or dispersion. The release composition of the present invention is compatible with coagulants currently used in the production of rubber articles. The combination of the coagulant and release composition into the same processing step, allows manufacturers to use the release composition in current processes without the need for costly modifications or the need for additional processing steps.
In addition to being useful as an outside coating on a glove, the polymer composition of the present invention is useful as a release agent in other industries, including but not limited to: injection molding, C-V joints, and in the manufacture of tires, synthetic gloves, and other rubber articles.
Articles formed using the release composition of the present invention possess good anti-blocking properties. This property prevents adhesion between articles, such as latex gloves, which are packed together, but must be easily separated from each other for use.
The following examples are presented to further illustrate and explain the present invention and should not be taken as limiting in any regard.
Method Axe2x80x94Making a Latex Glove with a Pre-treated Former
A latex glove can be made by:
(a) Immersing a glove former in a solution or dispersion comprising a release composition comprising a water-borne polymer with Tg higher than xe2x88x9210xc2x0 C. formed from at least one hydrophobic monomer, and at least one hydrophilic monomer, producing a coated former;
(b) after drying, immersing the coated former into a conventional coagulant solution;
(c) immersing said coated mold into a natural rubber latex to coat the former with said latex;
(d) optionally, leaching the latex coated former in water to remove impurities from the rubber;
(e) immersing the latex coated former into a inner surface coating composition;
(f) curing the latex in an oven; and
(g) removing the finished glove from the former.
Method Bxe2x80x94Making a Latex Glove with a Formulated Coagulant
A latex glove can be made by:
(a) immersing a glove former in a coagulant solution containing a release composition comprising a water-borne polymer with Tg higher than xe2x88x9210xc2x0 C. formed from at least one hydrophobic monomer, and at least one hydrophilic monomer, producing a coated former;
(b) immersing said coated mold into a natural rubber latex to coat the former with said latex;
(c) optionally, leaching the latex coated former in water to remove impurities from the rubber;
(d) immersing the latex coated former into a inner surface coating composition;
(e) curing the latex in an oven; and
(f) removing the finished glove from the former.